The wide spread use of computers has resulted in many tasks that are automatically controlled, with unattended processing performed by the computer. In the event of utility or power system failure, it is desirable that the operating program of the computer be logically closed and the point of interruption in the program flow be recorded. Data being processed in the computer, at the time of power system interruption, should be stored so that a resumption of processing can take place when the utility returns. For example, if the power fails without any prior provision for orderly shutdown, the computer may be in the process of transmission of information to peripheral devices (e.g., a disk, printer etc.) at the moment failure occurs. The operator cannot be certain of how much of the information was transmitted successfully, and whether electrical irregularities, such as spikes, during failure may have altered the information. The operator cannot conveniently reconstruct the failure to determine whether the information transmitted to the peripheral device is correct and reliable. The status of the computer is therefore poorly defined, and the operator cannot be certain what would happen if he simply restarted the computer and hoped that it would restart at the point of interruption without losing information. The operator then can choose to gamble on an orderly restart or begin the entire computing operation over, if that is possible under the circumstances.
Major power losses are relatively rare in most places. Unfortunately, computers are typically sensitive to brief power failures lasting only a fraction of a second and to partial power failures that reduce the main power line voltage below the design limits of the computer. The computer may be vulnerable to power losses and fluctuations that are not even noticeable to a person who is watching the computer, since they may not significantly effect many other types of electrical devices. Both major and minor power losses can interrupt normal computer operations in a way that makes reconstruction and restoration of the computer's status nearly impossible, because it is difficult to determine the exact time, status, and failure sequence of the computer when the power loss occurred.
As a response to the extreme sensitivity of computer systems to electrical interruptions, uninterruptable power supplies (UPSs) have been developed for nearly all types of computers. For example, see, U.S. Pat. No. 4,757,505--Marrington et al., (Marrington or the '505). Marrington discloses a backup computer power system for powering and controlling a computer which is otherwise powered by a main power source, when then main power source fails or is disconnected.
The '505 patent discloses an auxiliary power supply for providing power to a computer. An instruction set residing in the computer controllably shuts down the computer after first allowing optional operator intervention prior to initiation of the programmed shutdown. The automatic shutdown of the computer is triggered by a signal from a status monitor when main or utility power fails. When the main power fails, a relay is actuated and the computer is connected to a battery. If the operator does not intervene within a predefined period of time (e.g., 5 minutes), the instruction set terminates the current activity of the computer, saves the software operating environment of the computer, and shuts down the computer.
A significant short-coming with the prior art system of the type described above is that the time during which the computer is allowed to operate, on battery power after a main power failure, is not maximized. In other words, absent restoration of the main power, the computer will automatically shut down regardless of the useful lifetime remaining in the battery. In many situations it would be desireable to allow the computer to continue processing information for as long as possible before the battery is depleted beyond the point at which an orderly shutdown could be achieved.
Thus, a primary object of the present invention is to provide a UPS-computer system in which the battery voltage can be observed during normal and emergency operation and can provide information that is useful in determining when to initiate an orderly shutdown of the computer.